As a known speed reduction or speed increasing apparatus, one in which two spur gears having an involute tooth profile is known. The spur gear having the involute tooth profile is heavily used in the speed reduction or speed increasing apparatus since its manufacture is easy and it is efficient. However, there is a limit to the difference in the number of teeth between two spur gears. Hence, there is a problem that the speed reduction ratio or speed increasing ratio cannot be increased.
In order to increase the speed reduction ratio or speed increasing ratio, a speed reduction or speed increasing apparatus has been developed in which a first crown gear and a second crown gear having different numbers of teeth from each other are caused to face each other, the first crown gear is inclined with respect to the second crown gear in such a manner that the first crown gear engages with the second crown gear, and the first crown gear is caused to undergo wave motion in such a manner that the location of contact moves. When the first crown gear is caused to undergo wave motion, the second crown gear makes rotations equal to a difference in the number of teeth relatively to the first crown gear whenever the location of contact makes one rotation. A large speed reduction ratio can be obtained by outputting rotations equal to the difference in the number of teeth between the gears. Conversely, a large speed increasing ratio can be obtained by rotating the second crown gear.
For example, Patent Literature 1 discloses a speed reduction apparatus including a first crown gear, a second crown gear, and an inclined cam, as this kind of speed reduction apparatus. The first crown gear and the second crown gear have different numbers of teeth, and face each other. The first crown gear is inclined by the inclined cam integral with an input shaft. When the first crown gear is inclined, the first crown gear and the second crown gear engage with each other at one point. The second crown gear is fixed to a housing. The first crown gear is supported by a spherical spline joint placed radially inward of the first crown gear in such a manner as to be capable of wave motion.
When the input shaft is rotated, the inclined cam integral with the input shaft causes the first crown gear to undergo wave motion while moving the location of contact with the second crown gear. The wave motion of the first crown gear causes the first crown gear to make rotations equal to the difference in the number of teeth with respect to the second crown gear. The rotation of the first crown gear is transmitted to an output shaft via the spherical spline joint.